2. Wil Wheaton (as himself)
As hilarious and antagonizing as his character is scarce, Wil Wheaton's transition from Sheldon's arch-nemesis to 'close personal friend' made for some of the funniest and most memorable episodes of the Big Bang Theory. Wil Wheaton's biting sarcasm and reaction to Sheldon brings out his frustrations, making them a comedy duo to be reckoned with. He also has one of those names that just has to be said in its entirety Though Wil (see, it doesn't sound right) appears on screen infrequently, his character has been a crucial inclusion in some of the bigger and long-running story lines. He assisted the break-up of Penny and Leonard after a tense game at the bowling alley and after years of trauma related to a comic book convention to which he did not show up, he finally signed his Wesley Crusher action figure for Sheldon. The Star Trek actor also featured in Sheldon & Amy's Fun With Flags web show during 'The Habitation Configuration', one of the most hilarious episodes of the latter seasons. He's only appeared in six episodes, but each one is unforgettable.